TECHNICAL LIBRARY

Simulation of an Organic Photovoltaic Cell (OPC) Using ATLAS

Introduction

In recent years, the investigation of Organic Light Emitting Diodes
(OLEDs) and photovoltaic devices based on small organic molecules and polymers
has attracted significant interest due to their potential for inexpensively generated
electricity. ATLAS has been used already to investigate OLEDs [1] and compound
material GaInP[2][3] devices.

In this article, we will present the use of
the ATLAS simulator for the analysis of a PiN organic photovoltaic cell based
on the organic material TPD, blend-ZnPc/C60(1:1),
C60.

Simulation Models

Optical ModelingThe Ray-Tracing Method (RTM), Transfer Matrix Method(TMM)
and Beam Propagation Method(BPM) were implemented in the ATLAS/Luminous simulation
package for optical
modeling. In this article, TMM was employed in which the amplitude of the electromagnetic
field vector is calculated by taking into account the optical constants n and
k.

Figure 1. Refractive index profile of a single-layer coating

To enable the transfer matrix method for calculation of intensity distribution
and photo-generation rate profiles in thin film detectors, the TR.MATRIX parameter
on the BEAM statement should be specified.

Light Absorption and Photo-Generation
The cumulative effects of the reflection coefficients, transmission coefficients,
and the integrated loss due to absorption over the ray path are saved for each
ray. The generation associated with each grid point can be calculated by integration
of the generation rate over the area of intersection between the ray and the
polygon associated with the grid point. For multi-spectral source, the generation
rate is given by:

where, 0 is the internal quantum efficiency.

P() is the
power spectral density
of the source.

L is a factor representing the cumulative loss due to reflections,
transmissions, and absorption over the ray path.

is the wavelength.

h is Planck’s constant.

c is the speed of light.

is the absorption coefficient given by

y is the depth of the device, where
x,y forms the two-dimensional mesh

Transport in Organic Materials
The following models are support in ATLAS/OTFT for
the disordered organic materials[4].

Density of state model for disordered organic
material

Hopping mobility model

Poole-Frenkel mobility model

Bimolecular Langevin recombination model

Simulation Structure and Results

Figure 2 shows the simulation structure used
in this article. It is a 5 layer device; the p type MeO-TPD, absorption layer
ZnPc:C60 and n type Rhodamine
B-C60 films are sandwiched between the Indium-Tin-Oxide(ITO) larger coated
on glass substrate and an Aluminum metal contact. The p and n type layers
are doped at 1e18cm-3, so the contact with both electrodes can be assumed
to be
ohmic.

The material parameters for the disordered organic materials are:
electron mobility of 2e-5 cm2/Vs hole mobility of 8e-5 cm2/Vs and
dopant density of 1e18 cm-3. For the active absorption layer, we
use a mobility of 2.5e-6 cm2/Vs
for holes and 5e-6 cm2/Vs for electrons. The Poole-Frenkel mobility
model
parameters (E0N.PFMOB and E0P.PFMOB) are specified as 2.5e5 V/cm.

The energy
levels are given in Table 1.

LUMO(eV)

HOMO(eV)

P-layer

2.4

5.1

i-layer

4.1

5.1

N-layer

4.1

6.0

Table 1. Energy levels for the PiN OPCs.

In the photovoltaic absorption layer, we calculate
the generation rate profile of charges resulting from absorption of the injected
light with
intensity
of 127 mW/cm2. The generation rate distribution is shown in Figure 3. The
p and
n type layers are not absorbing, so the generation rate is zero. The p
and n type transport layers do not contribute to the generation rate.

Figure 3. Generation rate profile of PiN solar cell.

The
distribution of free and trapped carriers and the electric field as a function
of position within the device are shown in Figures 4 and 5 Since
ohmic contacts
are assumed, the number of free carriers in the doped wide gap layer is
uniform
and equal to the dopant density. But in the intrinsic active absorption
layer, depletion regions are formed due to diffusion of the free majority
carriers
from the doped layers into the intrinsic layer. So the carrier distribution
has a profile of free carriers and trapped carriers from the balance of
recombination, generation, trapping and transport as shown in Figure 4.

Figure 4. Distribution of the free and trapped carriers without external
voltage under illumination.

Figure 5. Distribution of electric field under illumination of 127mW/cm2.

The
concentration of traps strongly influences the value of the current density
at positive and negative bias. This dependence is shown in Figure
6, at trap
densities ranging from 1e16 to 1e19 cm-3. As the trap concentration
is lowered, the current density reaches saturation due to low losses in
the
active layer.
Figure 7 shows the effects of illumination on current density.

Figure 6. I-V characteristics due to the concentration of trapping states
from 1e16 to 1e19cm-3 in the blend layer.

The power
conversion efficiency of this photovoltaic cell under illumination conditions
depends on Jsc and Voc which are shown in Figure 8. The fill
factor is given by:

In this structure, Jsc is 8.9mA/cm2, Voc is 0.63V and
max(JpVp) is 2.32 mW/cm2, so the FF is 41.4%

The power conversion efficiency
is 1.83%

Conclusion

An organic photovoltaic cell has been analyzed using the ATLAS simulator
to simulate the response of a blended photovoltaic device to incident solar
light.
This model is readily extended to a wide range of disordered organic materials.